What was the cause of the fourth crusade?
The truce between Richard the Lion-Hearted and Saladin did not last so Pope Innocent III launched the 4th crusade.
The fourth crusade was a children's crusade. The children set off to take back Jerusalem which was under control of the seljuk turks. The children never made to Jerusalem but instead attacked constantinople on accident. They mistook it for Jerusalem. The children lost and the ones who escaped were tricked into being sold into slavery.